Many cancers are characterized by disruptions in cellular signaling pathways that lead to uncontrolled growth and proliferation of cancerous cells. Receptors, including hormone receptors, play a pivotal role in these signaling pathways, transmitting extracellular molecular signals into the cytoplasm and/or nucleus of a cell. Among such receptors is the nuclear hormone receptor estrogen receptor α (ER α), which acts as a transcription factor controlling gene expression mediated by estrogen signaling.
It has long been recognized that ER α plays a central role in the development and progression of breast cancer. Breast cancer is the second most common form of cancer in women, with about 192,000 new cases annually in the United States alone. It is estimated than nearly 40,000 women will die from the disease each year (Source: American Cancer Society). More than half of all breast cancers express ER α, and about two-thirds of these cancers respond to anti-estrogen therapy, emphasizing the importance of this receptor in disease progression. See generally, Osborne er at., Clin. Cancer 7(12): 4338s–4342s (2001).
The transcriptional activity of ER α is mediated through two distinct domains, transcription activation functions 1 and 2 (AF-1 and AF-2), which can act either independently or synergistically, in a promoter- or cell-dependent manner. See, e.g. Beato et al. Cell 83: 851–857 (1995); Tsai et al., Annu. Rev. Biochem. 63: 451–486 (1994). While estrogen binding controls AF-2 transcriptional activity, it has been shown that phosphorylation of ER α at one of three sites (serine 104 and/or 106,118, and/or 167) in the 180 amino acid N-terminal domain comprising AF-1 mediates the estrogen-independent transcriptional activity of this domain. See e.g. Le Goff et al., J. Biol. Chem. 269(6): 4458–66 (1994). In particular, phosphorylation of serine 118, which can occur via the MAPK pathway, has been shown to be critical for the AF-1 activity of ER α. See, e.g. Kato et al., Science 270: 1491–94 (1995); Ali et al., EMBO J. 12(3): 1153–60 (1993); Joel et al., Mol. Endocrinol. 9(8): 1041–52 (1995); Castano et al. J. Steroid Biochem. Mol. Biol. 65: 101–110 (1998). Antibodies to ER α have been described (see, e.g. Kobayashi et al., Breast Cancer 7(2): 136–41 (2000); Rost et al., Steroids 65(8): 429–36 (2000); Tyulmenkov et al., Steroids 65: 505–512 (2000)), and a phospho-specific polyclonal antibody to ER α (Ser118) is commercially available (Santa Cruz Biotechnology, Inc., Cat. No. sc-12915).
Given the prevalence of ER α activation in breast cancers, this receptor has long been a target for inhibitory therapies. Tamoxifen, a systemic ER inhibitor (acting to suppress AF-1 activity), has been the therapy of choice for ER-positive breast cancer for many decades. More recently, second-generation inhibitors, such as raloxifene, have entered clinical development. Unfortunately, although about fifty percent of patients with advance ER-positive breast cancer initially respond to tamoxifen therapy, the disease ultimate acquires tamoxifen-resistance through mechanisms that are not presently well understood. See, e.g. Horwitz, Breast Cancer Res. 1: 5–6 (1999). Despite the lack of a clear mechanism of tamoxifen-resistance, it has been postulated that steroid-independent phosphorylation of ER mutants may be a contributing factor. Bunone et al. EMBO J. 15(9): 2174–83 (1996). More recent reports indicate that acquisition of tamoxifen-resistance in ER-positive breast cancer is likely to involve multiple, interrelated signaling pathways in an estrogen-independent manner. See, e.g. Clarke et al., J. Steroid Biochem. Mol. Biol. 76: 71–84 (2001); Dorssers et al., Drugs 61(12): 1721–33 (2001). Indeed, it has been reported that the efficient phosphorylation of ER α at serine 118 requires a ligand-regulated interaction between CDK and AF-2. See, e.g. Chen et al., Mol. Cell 6(1):127–37 (2000).
Accordingly, given the importance of ER α in breast cancer progression, there remains a need for new and improved reagents, such as monoclonal antibodies, capable of specifically detecting phosphorylation (and hence, activation) of ER α at serine 118. Since phosphorylation-dependent activation of ER α is associated with diseases such as breast cancer, reagents enabling the specific detection of ER α activation would be useful tools for research and clinical applications. ER α (Ser118)-specific monoclonal antibodies would be desirable for studying the mechanisms of resistance to therapeutics like tamoxifen, as well as for applications, such as diagnostic assays, where reagent purity and consistency are paramount.